UDF

Hi How can I write a UDF of the boundary condition T(t)=Tm+Asin(wt), where T is the temperature, t=time, Tm=mean temaperature, A= amplitude=DeltaT/2, w=2pi/P=angular freequency, P=24 for the fluent....

That's because you have not define sin function in the library. You need to write include math.h as well afer the include udf.h. This will define the sin function. Only UDF library is noy enough to recognise it.